1. Field of the Invention
The present invention relates generally to advanced technology thermal battery systems. More particularly, the present invention relates to sodium-sulfur thermal batteries for use in providing a high-power density electrical energy source.
2. Description of Related Art
The sodium-sulfur battery was first introduced in the mid 1960's Since that time, there has been a great deal of interest in developing cell designs which are suitable for a wide variety of applications. Batteries which have been under development include those for use in automobiles and train locomotives. One such battery is described by J. L. Sudworth in the publication entitled "Sodium/Sulfur Batteries for Rail Traction," in the Record the Tenth Intersociety Energy Conversion Engineering Conference, 1975, pages 616-620. Cell designs have also been investigated for producing batteries for storage of electricity for delayed use in order to level out the production rate of electricity and for space systems requiring high energy density. The sodium-sulfur battery is used as a secondary, that is, rechargeable battery. Its use as a primary (one-time discharge) battery would be unwarranted because of the cost, complexity and fragility involved in edge-sealing and incorporating a ceramic solid electrolyte into a battery design. In addition, there are other relatively inexpensive primary batteries of higher power density available in the marketplace.
The typical sodium-sulfur electrochemical cell includes a molten metallic sodium anode, a sodium ion conducting ceramic solid electrolyte and a molten sulfur electrode. The sodium-sulfur cell usually operates at a relatively high temperature (300.degree.-400.degree. C.) in order to maintain not only the sulfur and sodium, but also their reaction products, in a molten state. The solid electrolyte is a critical part of the cell configuration because it must also provide separation of the liquid sodium from the liquid sulfur in order to prevent catastrophic cell failure. Finding a suitable solid electrolyte has been a difficult task because of the high conductivity required for a high power density battery.
Solid electrolytes which have been used in sodium-sulfur batteries include beta"-alumina and other sodium ion conducting ceramic or glass. Beta"-alumina has become the most popular solid electrolyte. However, a problem with all of these solid electrolytes is that they suffer from relatively low conductivity and have coefficients of thermal expansion which are not well matched to other materials used in making the cell. Accordingly, the present solid separation cell configurations are fragile and are limited to relatively low power outputs. In addition, the differences in thermal expansion between the ceramic material and other cell elements make it difficult to provide a seal around the edges of the ceramic separator. Further, the differential stresses present in the sodium-sulfur cell during operation may weaken the solid electrolyte resulting in the formation of cracks or other structural failures.
In view of the above problems associated with the present solid electrolytes, there is a continuing need to develop sodium-sulfur battery configurations wherein higher levels of conductivity are achieved through the electrolyte. In addition, new separator designs are necessary which do not have thermal expansion mismatches or sensitivity to catastrophic failure. Further there is a need for a primary battery which has high power density, long shelf-life, ruggedness, and discharge times ranging from seconds to several hours.